This invention relates generally to the field of fluid delivery systems and more particularly, but not by way of limitation, to delivery of liquefied gases from a point of sale delivery vessel while maintaining a predetermined vessel pressure so as to improve liquid transfer.
Liquefied gases, such as liquefied petroleum gas (LPG) or anhydrous ammonia, are often stored in vessels for on demand use by a customer. These liquids are referred to as liquefied gases because at standard temperature and pressure, these substances are gaseous. Thus, to transport large quantities of the liquefied gases, sometimes referred to as delivery fluids, the substances are pressurized or refrigerated to maintain the substances liquefied.
From time to time, a customer vessel or tank is refilled using a portable liquid delivery system. The liquid delivery system includes a liquid delivery vehicle having a pressurized delivery tank and associated equipment to transfer the delivery fluid. A typical fluid transfer from such a vehicle involves connecting a hose from the delivery tank to the customer tank and pumping delivery fluid from the delivery tank to the customer tank while metering the flow to determine the total amount of delivery fluid transferred to the customer tank.
Because the delivery truck comprises the point of sale, it is generally undesirable to connect a second hose from the vapor space of the customer tank to the vapor space of the delivery tank to maintain vessel pressure in the delivery tank. This arrangement allows some amount of vaporized delivery fluid to transfer back from the customer tank to the delivery tank. As a result, as liquid delivery fluid is drawn from the delivery tank, the pressure drops in the vapor space of the delivery tank and the liquid will boil to fill the vapor space to maintain an equilibrium state. This boiling, if sufficiently violent, can cause vapor to be drawn into the pump inlet, reducing delivery fluid transfer rate and causing cavitation, noise, vibration and ultimate damage to the pump, meter and hoses. This phenomenon becomes more likely as the delivery tank approaches an empty liquid level.
A solution to this problem has been proposed by Midwest Meter Company, Hampton, Iowa, USA, involving a shell-and-tube heat exchanger that receives a small amount of fluid from the delivery tank into a first conduit path within the heat exchanger. A different hot fluid, such as hot water supplied from the engine of the delivery vehicle, is passed through a second conduit path of the heat exchanger. The thermal transfer of heat from the second conduit path to the first conduit path vaporizes the inlet delivery fluid to produce an amount of vapor that is introduced into the vapor space of the delivery tank.
While generally operable, this approach has limitations. For one thing, the shell-and-tube heat exchanger is relatively large, relies on pressurized feed based on the internal pressure of the delivery tank, and incurs damage from such effects as extended vibration from vehicle movement. Such damage can cause cross-contamination and reduced efficiency over time. For another thing, this system is also limited in terms of the ability to accommodate a wide range of pressure and temperature ranges, as well as different pumping rates.
Accordingly, there is a need for improvements in the art of delivering pressurized fluids from a portable delivery system, and it is to such improvements that the present invention is directed.
A delivery system is provided for transferring a delivery fluid from a delivery tank to a customer tank while maintaining a desired vessel pressure in the delivery tank. The delivery system includes a piping system between the delivery tank and the customer tank and a pump to transport the delivery fluid through the piping system. The delivery system also includes a slip-stream junction where part of the flow downstream of the pump is diverted back to the delivery tank. The flow that is diverted back to the delivery tank passes through a variable flow control valve and a heat exchanger, where the delivery fluid exchanges heat with a hot heat exchanger fluid to vaporize the delivery fluid diverted back to the delivery tank. A heat exchanger fluid source provides the heat exchanger fluid to a heat exchanger pump assembly, which transports the heat exchanger fluid through the heat exchanger.
The vessel pressure in the delivery tank is controlled by: (1) adjusting the flow rate of the delivery fluid fed back to the delivery tank by adjusting the variable flow control valve; and (2) adjusting the rate of flow of heat exchanger fluid through the heat exchanger pump assembly. A programmable controller controls the adjustments of the variable flow control valve and the heat exchanger pump flow rate in response to signals received from control elements. The control elements may be a pressure sensor in a vapor space of the delivery tank, a temperature sensor in the vapor space of the delivery tank, a flow meter located in the piping system between the pump assembly and the slip-stream junction, a vibration detector attached to the delivery tank, or some suitable combination of these control elements.
These and various other features as well as advantages which characterize the claimed invention will become apparent upon reading the following detailed description and upon reviewing the associated drawings and appended claims.